Introduce chassisd to monitor status of cards on chassis

[mprabhu-nokia]

Introducing chassisd to monitor status of cards on a modular chassis

HLD: sonic-net/SONiC#646

-What I did
Introducing a new process to monitor status of control, line and fabric cards.

-How I did it
Support of monitoring of line-cards and fabric-cards. This runs in the main thread periodically.
It updates the STATE_DB with the status information. 'show platform chassis-modules' will read from the STATE_DB

Support of handling configuration of moving the cards toadministratively up/down state. The handling happens as part
of a separate thread that waits on select() for config event from a CHASSIS_MODULE table in CONFIG_DB.

-How I verified it

[lgtm-com]

This pull request introduces 3 alerts when merging e855529 into 1aaffcc - view on LGTM.com

new alerts:

• 2 for Unused import
• 1 for Unreachable code

[lgtm-com]

This pull request introduces 4 alerts when merging d82046b into 8507085 - view on LGTM.com

new alerts:

• 2 for Unused import
• 1 for Unused local variable
• 1 for Unreachable code

[lguohan]

can we start with python3, also what is the unit test that can be used to test it?

[jleveque]

Please also add unit tests for chassisd

[lgtm-com]

This pull request introduces 5 alerts when merging 2a1f40b into 61ed24e - view on LGTM.com

new alerts:

• 2 for Unused local variable
• 2 for Unused import
• 1 for Unreachable code

[mprabhu-nokia]

Please also add unit tests for chassisd

Added UT interfacing with Mock classes.

[mprabhu-nokia]

plugins: cov-2.6.0
collected 8 items

tests/test_chassisd.py ........                                                                                                                                                        [100%]

----------- coverage: platform linux, python 3.7.3-final-0 -----------
Name               Stmts   Miss  Cover
--------------------------------------
scripts/chassisd     177     64    64%
Coverage HTML written to dir htmlcov
Coverage XML written to file coverage.xml

The missing coverage is mainly around the area of:

• init, signal handling of the main loop
• async subscription handling of the config DB. select() handling etc.

However, the functional area has been separated to classes and fully covered.

[jleveque]

@liat-grozovik: Can someone from your team please review?

@Staphylo: Can you also review, please?

[keboliu]

I think this daemon should only for the chassis system, so how to prevent it from running on a 1U system?

[mprabhu-nokia]

I think this daemon should only for the chassis system, so how to prevent it from running on a 1U system?

With the current implementation, the process will start but gracefully exit if not running for modular chassis. This is not part of critical processes, so should not affect PMON container.
Going forward, one option would be for vendors to add skip_chassisd in their pmon_daemon_control.json file for 1 RU SKUs.